Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

• Published in: Journal of materials science & technology Vol. 34; no. 2; pp. 265 - 276
• Main Authors: Kim, Sang-Hoon, Lee, Jong Un, Kim, Ye Jin, Bae, Jun Ho, You, Bong Sun, Park, Sung Hyuk
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-02-2018
• Subjects: Aging; Grain refinement; Magnesium; Precipitation; Tensile properties; Aging; Precipitation; Magnesium; Grain refinement; Tensile properties
• ISSN: 1005-0302; 1941-1162
• DOI: 10.1016/j.jmst.2017.11.019

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•Effects of grain refinement on the aging behavior of cast AZ92 alloy were studied.•Discontinuous precipitation is accelerated by increased grain boundaries.•Continuous precipitation is quickly completed by a reduced chemical driving force.•The peak-aging time reduces significantly to one-third by grain refinement.•Grain refinement improves the strength of homogenized and peak-aged alloys.
This study demonstrates that the precipitation behavior of β-Mg17Al12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.